This invention relates to a fuel supply control system for internal combustion engines, and more particularly to a system of this kind which is adapted to properly control the air-fuel ratio of a mixture of fuel supplied to the engine, during and immediately after acceleration of the engine.
There is generally known a fuel supply control method for internal combustion engines, which utilizes electronic means to subject a basic value Ti of the fuel injection period, which is determined from the engine rotational speed and absolute pressure within the engine intake pipe, to multiplication and/or addition by correction values and/or correction coefficients determined from engine operating parameters, such as the engine rotational speed, the intake pipe absolute pressure, the engine coolant temperature, the throttle valve opening and the concentration of an ingredient (oxygen) contained in exhaust gases emitted from the engine, to thereby determine the valve opening period for fuel injection valves and hence control the air-fuel ratio of a mixture supplied to the engine.
In the generally known fuel supply control method, it is also known, e.g. from Japanese Provisional Patent Publication (Kokai) No. 60-3458, to add an accelerating fuel increment T.sub.ACC determined from a rate of change in the opening degree of the throttle valve to the basic value Ti of the fuel injection period at the beginning of acceleration of the engine, in order to improve the accelerability of the engine.
Further, in the generally known method, it is also known, e.g. from Japanese Provisional Patent Publication (kokai) No. 60-60234 to set the accelerating fuel increment T.sub.ACC in such a manner that it is first set in accordance with the rate of change in the opening degree of the throttle valve during acceleration of the engine, and progressively decreased at a predetermined rate immediately after the acceleration, thereby improving the accelerability, drivability, etc. of the engine.
According to the former method, however, immediately when the engine enters an accelerating state, the accelerating fuel increment T.sub.ACC is determined from various engine operating parameters, including not only the rate of change in the throttle valve opening degree, but also the engine rotational speed, and whether or not fuel cut was effected just before the acceleration. However, once the engine has entered the accelerating state, the accelerating fuel increment T.sub.ACC is determined solely from the rate of change in the throttle valve opening degree.
According to the latter method, on the other hand, the accelerating fuel increment T.sub.ACC is set such that it is progressively decreased at a predetermined rate independently of engine operating parameters, immediately after the acceleration of the engine.
Particularly, in the both methods, the accelerating fuel increment T.sub.ACC is set independently of the intake pipe absolute pressure, the amount of air supplied to the engine, and the basic value Ti of the fuel injection period.
This will be explained in details with reference to (a) to (d) of FIG. 8. When the throttle valve opening degree .theta..sub.TH is increased to demand acceleration of the engine, as shown in (a) of FIG. 8, the intake pipe absolute pressure P.sub.BA increases with a certain delay, so that the basic value Ti of fuel injection amount determined from the intake pipe absolute pressure P.sub.BA increases with delay, as shown in (b) of FIG. 8. Since the basic value Ti is usually set almost in proportion to the intake pipe absolute pressure P.sub.BA, it increases along almost the same rise curve as that of the intake pipe absolute pressure P.sub.BA.
In the known methods, on the other hand, the accelerating fuel increment T.sub.ACC is set such that it changes in accordance with the rate of change in the throttle valve opening degree, regardless of change in the basic value Ti of the fuel injection period shown in (b) of FIG. 8, as shown by the solid line in (c) of FIG. 8.
Therefore, the fuel injection period T.sub.OUT (hence the fuel injection amount), which is obtained by correcting the basic value Ti by adding the accelerating fuel increment T.sub.ACC, changes as shown by the solid line in (d) of FIG. 8, whereby an excessive amount of fuel is supplied to the engine, with respect to an intake air amount G.sub.air actually supplied to the engine. More specifically, as the throttle valve opening degree .theta..sub.TH increases as shown by the solid line in (a) of FIG. 8, the intake air amount G.sub.air increases as shown in (e) of FIG. 8, so that the fuel amount supplied to the engine becomes excessive by an amount corresponding to the hatched area, resulting in degraded exhaust emission characteristics, degraded drivability, increased fuel consumption, etc.